Although I have never worked in a ticket office, there must be a fair chance that some of those who have had poor experiences are because they don't know exactly what they want, they are unable to articulate what they want, what they want is no longer available or never has been, or the conditions (or, in the case of Rangers/ Rovers, the area covered) have changed. I guess that when a TOC changes the area it covers, there is a chance that the area of a Rover/ Ranger will change too(?), particularly if there are sections it no longer operates in.
As an example of the latter, there is a Ranger ticket that I used to purchase pretty much annually, haven't used it for some years. I've looked today, still available with the same name but range widened (and prices hiked massively, presumably because of wider range). If I still lived in the same area and hadn't looked it up, I might be spending time explaining to the office staff that I just want to go to .... and getting frustrated as a result.
The nudging point is well made, we cannot expect the staff to be fully aware of every rail option. I once wanted a ticket for a journey I had previously been on, when told the price, I indicated that it had previously been cheaper and started explaining more, when the staff member said that I would be travelling via ..., then we were on the same track. I had stipulated the TOC for that part of the journey when it was easier for him if I gave a place on route. Sometimes as (potential) passengers we don't help ourselves.
There will be ticket office staff who shouldn't be in the job but then there are in every profession.